It has been known for years that many bacteria require iron for growth. At least some types of bacteria and fungi obtain the iron they need by producing special compounds termed "siderophores" (Greek for "iron bearers") which are relatively low molecular-weight (less than about 1000 daltons) iron-binding ("iron chelating") compounds. Generally, siderophores are ferric-specific ligands, the natural purpose of which is to supply iron to the microorganism cells. Each of the several species of siderophores is a key component in the iron high-affinity system of the respective microorganism that includes specific membrane-associated receptors.
Representative siderophores include phenolate compounds such as "agrobactin" from Agrobacterium tumefaciens and "pseudobactin" from Pseudomonas, and hydroxamates such as "schizokinin" from Bacillus megaterium and ferrioxamines from Actinomyces. Siderophores produced by fungi include hydroxamates such as ferrichromes from Penicillium species, rhodotorulic acids from Rhodoturula, and other hydroxamates from certain Ectomycorrhiza species. See Neilands, Ann. Rev. Biochem. 50:715-731 (1981).
Host animals such as mammals and fish produce iron-binding proteins, including ferritin, transferrin, and hemosiderin, which tightly sequester ionic iron in the body. As a result, unbound or "free" iron (as ferrous or ferric ions) is present only at very low concentrations in a healthy host's plasma and other body fluids.
Siderophore production enables invading bacteria to successfully compete with the host's iron-binding proteins for iron in the host's body that would otherwise be unavailable to the bacteria. Without an ability to wrest bound iron from the host, bacteria would be unable to proliferate sufficiently to cause disease. Hence, production of siderophores is a key to bacterial pathogenicity.
A number of diseases in humans are demonstrative of the toxicity of free iron in the body. In general, the term "hypersiderosis" represents any of several disease conditions in which the normal iron-carrying capacity of a person's blood and tissue proteins is exceeded and pathological effects due to iron overload are manifest. In such conditions, the excess iron can become deposited in various tissues, such as the myocardium and liver.
Acute iron intoxication usually results from accidental over-ingestion of iron supplements, particularly by young children. Industrial accidents can also result in acute iron intoxication.
Chronic iron overload encompasses a variety of diseases where iron accumulates in the body due to various causes. For example, intestinal control of iron absorption may be ineffective so that inappropriate amounts of dietary iron are allowed to enter the body (e.g., idiopathic hemochromatosis and anemias with ineffective erythropoiesis). In such cases, iron overload develops even when a normal diet is consumed. Hemochromatosis can also occur in alcoholics with cirrhosis. Long-term exposure to a diet containing excessive iron can lead to iron overload in otherwise normal subjects (dietary iron overload). In addition, large amounts of parenteral preparations of iron inappropriately prescribed, or repeated blood transfusions for refractory anemias, may result in the accumulation of excess iron in the body (transfusional siderosis). The iron liberated from the transfused cells cannot be excreted and it accumulates in the cells of the reticuloendothelial system and in cardiac muscle, kidneys, thyroid gland and adrenal gland. Changes in iron distribution from the primary reticuloendothelial iron to parenchymal iron overload are ascribed to the high saturation of transferrin, which provides favorable conditions for uptake of iron by parenchymal cells. Free transferrin thus protects the tissues from siderosis.
An example of a hereditary disease characterized by chronic iron overload is Cooley's anemia (thallasemia major), where congestive heart failure often precedes rapid deterioration and death of the untreated patient almost always in early infancy.
Electrocardiogram abnormalities are the most frequent manifestations of the cardiomyopathy of hemochromatosis. In descending order of frequency, these are T-wave flattening and inversion, low-voltage tracings, arrhythmias both superventricular (notable auricular fibrillation) and/or ventricular (premature ventricular contractions which may precede ventricular tachycardia or ventricular fibrillation). Congestive heart failure is rarer, but may be fatal, especially in young subjects. Postmortem examination of the heart shows fibrosis and hemosiderin deposits which are greater in the ventricles than in the atria, greater on the left side than the right side, and greater in the epicentrum than in the endocardium. Iron chelation therapy offers the possibility of alleviating this harmful and potentially lethal accumulation of iron in cardiac tissue.
Virtually the only iron chelator or siderophore currently in pharmacological use is deferoxamine (DESFERAL from CIBA Pharmaceuticals; U.S. Pat. Nos. 3,118,823 and 3,153,621). Deferoxamine was originally isolated from Streptomyces pilosus. This drug chelates iron by forming a stable complex with an iron atom. The complex prevents the iron from entering into further chemical reactions. The drug has a high affinity for ferric iron (K.sub.a =10.sup.31) coupled with a very low affinity for calcium (K.sub.a =10.sup.2). Deferoxamine wrests iron from ferritin and hemosiderin but not readily from transferrin and substantially not at all from cytochromes and hemoglobin. Theoretically, deferoxamine is capable of binding about 8.5 parts by weight of ferric iron. The molecular weight of deferoxamine is 657 g/mol.
Deferoxamine has improved the prognosis for iron-overload patients. However, this drug has certain drawbacks. First, the drug is prone to acid hydrolysis and poorly absorbed after oral administration, requiring parenteral administration, which is particularly inconvenient for long-term therapy. Second, it becomes effective when the body's iron load is at least about ten times normal, which is a level at which iron-binding proteins in the body are saturated and toxic free iron is circulating in the body. Third, it is expensive to produce. At the present time, the cost of sufficient deferoxamine for a year's treatment of chronic iron overload is several thousand dollars. Fourth, deferoxamine is toxic and can cause a number of reactions, including allergic reactions: pruritis, wheals, rash, and anaphylaxis; and dysuria, gastrointestinal symptoms, diarrhea, fever, leg cramps, hypotension, and tachycardia. Intravenous LD.sub.50 values are 287 mg/kg in mice; 329 mg/kg in rats.
Another major disadvantage of deferoxamine is its use by certain microorganisms to enhance their pathogenicity in humans. For example, iron overload increases the susceptibility of patients to Yersinia enterocolitica infections. In some cases, treatment with deferoxamine has enhanced this susceptibility, resulting in generalized infections by providing this bacterium with a siderophore otherwise missing. In such cases, deferoxamine treatment must be discontinued until the infection is resolved.
Examples of siderophores that have not achieved the pharmaceutical popularity of deferoxamine include catechol derivatives as disclosed in U.S. Pat. Nos. 4,530,963 and 4,585,559 to DeVoe et al., and hydroxypyridone derivatives as disclosed in Hider et al., U.S. Pat. No. 4,666,927.
Hence, there is a need for a new pharmacological method for reducing the concentration of ferric iron in the body, for treating iron overload and related diseases, particularly by using a new siderophore that is relatively non-toxic, producible at low cost, and not utilizable by any known human pathogens.